(383au) Solubility Investigation of Niobium Compounds in Alkaline Medium

Due to its complexity, the chemistry of niobium has been under investigated ¹, and henceforth solubility data of niobium compounds in aqueous media are very scarce in the literature and the knowledge is mostly in the hands of the industry. Speciation diagrams are the most employed tool to study aqueous systems and evaluate the effect of pH and total niobium concentration on the species formation.

In pure aqueous media, niobium as Nb(V) is stable in the water stability field (Jo et al., 2022). Its complexes can exist in alkaline and acidic media with a tendency to hydrolyze due to their low stability in aqueous solutions and strong oxygen bonds ^2,3. Additionally, the complexes of niobium produce insoluble oxides with amphoteric behaviour in both acidic and basic conditions. These oxides produce anionic species above pH 4 and cationic species below pH 4 ⁴.

In alkaline medium, niobium forms Lindqvist type ions H_xM₆O₁₉^(x-8) (M= metal) and the formation of the cluster M₆O₁₉ has been reported ^5,6. This structure is related to hexaniobate salts H_xNb₆O₁₉^(x-8) which are dominant species for Nb(V) in an aqueous medium with pH ≥ 8 ^3,7,8. Elements such as potassium, sodium and lithium can be introduced to this structure and produce new compounds.

Potassium niobate as the most stable phase – KNbO₃ – is a perovskite-type compound and its crystalline structure depends on the temperature of synthesis, and it can be cubic, tetragonal, orthorhombic, and rhombic. At atmospheric temperature and pressure, potassium niobate has an orthorhombic structure ⁹. These properties may influence the behaviour of this compound in aqueous media. Depending on the synthesis path of niobates, its crystallization could involve the formation of metastable phases which are later changed to the most stable phase. In this case, a soluble metastable phase may be formed before the final stable phase is reached.

In acid medium niobium may form niobic acid, a hydrous niobium oxide (Nb₂O₅.nH₂O). Niobic acid can be produced when acidifying alkaline solutions of Nb or by adjusting the pH of acidic solutions of Nb, such as neutralizing solutions of NbCl₅ ³.

This study aimed to investigate the solubility of potassium niobate and niobic acid in aqueous and alkaline conditions under varying temperatures. Potassium niobate was synthesized from an alkaline liquor by cooling crystallization, as described in a previous study ¹⁰. Niobic acid was used as the commercial solid.

Methods

The potassium niobate was synthesized from Fe-Nb alloy fines, as shown in a previous study ¹⁰. The niobic acid HY-340 was obtained from CBMM (Companhia Brasileira de Metalurgia e Mineração). The phases of the solids were identified by XRD and their chemical composition by XRF.

Crystal 16 (Technobis^® – software v. 2.3.2.5300) was used to carry out the solubility measurements in different temperatures and concentrations of potassium niobate and niobic acid in KOH solution (0 to 1 mol/L). Different masses of the niobium compounds were weighed into 1 ml vials, to which deionized water of KOH solution was later added. The vials were place in the equipment and heated following a temperature ramp from 20 to 70°C, with a heating rate of 0.1°C/min. A stirring speed of 700 rpm was used in all the experiments to ensure uniform mixing. The data was collected following the transmission line from 0 % to 100 %.

OLI Studio Stream Analyser v. 11 was used to calculate speciation, and phase equilibria in multicomponent systems and to predict the solubility of the solids, under the available database. The thermodynamic framework AQ (aqueous model) and MSE (Mixed Solvent Electrolyte) were used.

Results

The speciation diagram of niobium in aqueous media was the first step to understanding the system and was also essential to elucidate the formation of niobium complexes. OLI Studio which was used in modelling the niobium system, considered only the stable phase KNbO₃ in in its database and lacks some additional potassium niobate phases.

The XRD (Figure 1) of synthesized potassium niobate shows the presence of two dominant phases KNbO₃ and K₄Nb₆O₁₇, with K₄Nb₆O₁₇ being the predominant phase. This indicates that the crystallization of KNbO₃ may occur via a metastable K₄Nb₆O₁₇ phase in the first stage. Consequently, these phases could exhibit different solubilities in the presented solutions.

Crystal 16 showed the dissolution of the solid with the increase in temperature over time and presented a consistent behaviour for both crystallized potassium niobate and niobic acid.

The solubility of potassium niobate and niobic acid were predicted considering the temperature and concentration of KOH in aqueous media. According to OLI, the KNbO₃ in water has low solubility and shows a maximum solubility at 90°C, followed by a slight decrease in the solubility. This is distinct for the solubility of Nb₂O₅.nH₂O in water, which slightly increases with temperature. When the solubility of these solids were evaluated in KOH solution, the niobic acid showed an increase in solubility with the increase of concentration of KOH solution from 0.5 mol/L to 1.0 mol/L. A different behavior was observed in potassium niobate, to which the solubility decreased with the increase in concentration of KOH solution, from 0.1 mol/L to 1.0 mol/L, as shown in Figure 2.

Conclusions

The study of the solubility of potassium niobate and niobic acid in an alkaline medium using Crystal 16 showed great potential for the measurement of the solubility of both solids, essential for the development of new niobium applications and products. This knowledge, which is scarce in the literature, will allow advancements in the chemistry of niobium.

The relevance and impact of this project for scientific, technological, and innovation development are the generation of specific solubility-related knowledge for niobium compounds and provide the potential to develop processes for secondary materials for new applications.

References

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